Ocean dead zones are regions where water becomes stripped of its dissolved oxygen. These “hypoxic” (low oxygen) and "anoxic" (no oxygen) conditions can prove lethal for many marine species, changing the biology and chemistry of the ocean. In a few places throughout the world, dead zones occur throughout the year, but summer storms can intensify agricultural runoff, resulting in big phytoplankton blooms that trigger the phenomena.
Phytoplankton are critical to life and are particularly important marine organisms. These microscopic aquatic plants are dispersed throughout the world’s oceans. The plants’ distribution is driven by available light, the presence of nutrients, and physical processes like ocean circulation and upwelling.
"It’s safe to say, with a few exceptions, that all life in the ocean ultimately depends on phytoplankton for its nutrition," notes NASA oceanographer Gene Feldman, Goddard Space Flight Center, Greenbelt, Md.
Under certain conditions, excessive phytoplankton growth can result in a dead zone. Dead zones form when large quantities of organic matter, present from the excessive blooms of phytoplankton at the surface, sink to the bottom. After the organic matter or dead phytoplankton sink, bacteria break the dead or decaying phytoplankton down in a process known as aerobic decomposition, releasing carbon dioxide but absorbing oxygen as they work. The resulting low oxygen conditions can cover expansive areas, killing the oxygen-dependent aquatic species, such as fish, that cannot escape their reach, or driving them out of their habitat.
Summer storms intensify the runoff of fertilizers from lawns and farmland, which seep into the rivers and streams that comprise a local watershed and provide a jolt of nutrients to phytoplankton that live along the shore. Excessive nutrients from human activity are one reason many dead zones occur at the mouth of large rivers and in the bays along the shore.